1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine and, more particularly, to an intake system intake port structure for an internal combustion engine which produces vertical intake air turbulence in a combustion chamber.
2. Description of Related Art
Internal combustion engines have intake ports through which intake air is introduced into a combustion chamber. To obtain high combustion efficiency, it is necessary to produce strong "vertical turbulence" of intake air in a combustion chamber during an intake stroke, without using what is known as a "mask member," and/or to produce induction turbulence or swirling of intake air in a combustion chamber during an intake stroke, without using a mask member. Such vertical intake air turbulence and/or swirling of intake air is produced by providing an unbalanced distribution of intake air through an intake passage of an intake system including an intake port. In such an intake system, if an intake port has a circular cross section throughout its whole length and is curved or bent in a direction of intake air flow, a spiral motion of intake flow is undesirably generated by a secondary component of intake air flow at the curved or bent portion of the intake port. In particular, if the curved or bent portion of the intake port is large in length, because of the spiral motion of the intake air flow, it is difficult to provide an intended intake air distribution suitable for generating a desired vertical turbulence of intake air in the combustion chamber during an intake stroke.
To provide an unbalanced distribution of intake air, it has been proposed to form such a curved intake port which has a particular cross section uniformly configured along the whole length The cross section includes an upper or outer half section (which is a half section on an outer side of a center of the cross section as viewed from a center of curvature of the curved intake port) and a lower or inner half section (which is a half section on an inner side of a center of the cross section as viewed from a center of curvature of the curved intake port) smaller in width than the upper half section. The intake port, having a cross section thus configured, causes intake air to flow at a flow rate higher at the outer half section of the intake port than at the inner half section of the intake port. This produces intake air turbulence in the combustion chamber during an intake stroke. Such an intake port is known from, for instance, Japanese Unexamined Utility Model Publication No. 63-113,748.
The intake port described in this publication, however, causes a large loss in pressure of intake air at a transitional portion between the deformed cross section of the intake port and a round entrance to the combustion chamber. Such pressure loss leads to a drop in charging efficiency.
In an internal combustion engine having two intake ports per cylinder, intake air flows introduced through these two intake ports are directed in opposite circumferential directions in a combustion chamber. Such intake air flows interrupt the stable production of intake air swirling.